In optics, gratings with a spatially varying periodic refractive index provide general means of spectral control of light propagation in optical materials. Such refractive index modulation in optical fibers may provide a fiber Bragg grating (“FBG”), widely employed in sensing and Telecom applications as a narrow-band spectral filter. Another manifestation of the fiber-based gratings is the long period grating (“LPG”) which has a much larger period of index modulation compared with FBGs. These grating devices are prominent in current sensing and communication applications as fundamental filter or sensor components.
Several methods have been applied to fabricate Bragg and long-period gratings in optical fibers or planner waveguide circuits, but there has been only limited demonstration of such structures in three-dimensional (“3-D”) optical circuits.
For gratings in optical fibers, Hill et al. disclosed a FBG structure in U.S. Pat. No. 4,474,427 (1984), which exhibits a Bragg reflection peak only at the wavelength of the writing laser. U.S. Pat. No. 4,807,950 (1989) to Glenn et al. discloses a practical method of FBG fabrication by two-beam laser interference (holography) through the side of the fiber with an ultraviolet laser source. U.S. Pat. No. 5,104,209 (1992) to Hill et al. describes fiber grating fabrication by a point-by-point technique, where an ultraviolet laser beam is pre-shaped by narrow slit masks and flashed through the fiber cladding into the fiber core while the fiber is precisely moved between each laser exposure with respect to the mask. All of these methods require a pre-existing waveguide in which the external ultraviolet laser can interact with sufficient photosensitivity response and modify the refractive index change.
A further improvement to the point-by-point method is disclosed by Snitzer et al. in Canadian Patent No. 2,372,939 (1994) where an amplitude mask comprising a series of square apertures induces the laser light to interfere inside a nearby fiber. Hill et al. in U.S. Pat. No. 5,367,588 (1994) teach FBG fabrication by phase mask interference, which improves the optical exposure stability over the holographic interference technique. Both amplitude and phase mask techniques provide only one Bragg wavelength, and are therefore an inflexible fabrication method where multi-wavelength spectral responses are required. These mask techniques also require ultraviolet light excitation of a pre-existing waveguide (a photosensitive core).
Further, Albert et al. in U.S. Pat. No. 6,256,435 (2001) teach a method of forming Bragg gratings in a planar lightwave circuit (“PLC”) with UV laser light and a phasemask. This method also requires an existing waveguide, such as a Germanium-doped planar waveguide.
Ultrashort laser pulses with femtosecond to picosecond durations have been used to fabricate a broad range of optical devices including buried optical waveguides and gratings. For example, Mourou et al. in U.S. Pat. No. 5,656,186 (1997) describe ultrashort laser interactions with materials, but no devices such as waveguide or grating were described. Mihailov et al. in U.S. Pat. No. 6,993,221 (2006) teach the combination of ultrafast laser and phase mask for FBG fabrication. Kalachev et al. in Journal of Lightwave Technology 23, 8, 2568-2578 (2005) disclose a femtosecond ultraviolet light source (250 fs, 211 nm) method for fabricating a long period fiber grating with point-by-point exposure. As well, Martinez et al., in “Direct writing of fiber Bragg gratings by femtosecond laser”, Electron. Lett. 40, 19 (2004), describe point-by-point writing of FBGs with a femtosecond laser (150-fs duration, 1-kHz repetition rate). However, these techniques have only been demonstrated to be successful in a pre-existing waveguide (optical fiber) with no evidence of applicability in PLC or 3-D photonic circuits.
Ultrashort laser systems as well as other sources have also been applied for direct writing of two-dimensional (“2-D”) or 3-D photonic devices in various materials. For example, Davis et al. in “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21, 1729-1731 (1996) disclose a method of forming buried optical waveguides with ultrashort duration lasers. Definitive guiding of light in such structures was subsequently demonstrated by the same research group (K. Miura, Jianrong Qiu, H. Inouye, T. Mitsuyu, K. Hirao, Photowritten optical waveguides in various glasses with ultrashort pulse laser, Appl. Phys. Lett. 71, 3329-3331 (1997)), and then extended by Borrelli et al. in U.S. Pat. No. 6,977,137 (2005) to writing waveguides and other optical devices into three dimensions.
Short-pulse laser writing of waveguides in crystalline materials was demonstrated by Nolte et al. in “Waveguides produced by ultrashort laser pulses inside glasses and crystals”, Proc. of SPIE Vol 4637, 188-196 (2002), and “Femtosecond writing of high quality waveguide inside phosphate glasses and crystalline media using a bifocal approach”, Proc. of SPIE, vol. 5340, 164-171 (2004), and also disclosed recently in PCT Patent No. WO 2005/040874 to Khruschev et al. Khruschev et al. further propose a laser method of forming volume diffraction gratings by writing multiple parallel waveguides side by side.
In addition to the use of ultrashort (i.e. <10 ps) lasers to write buried waveguides and related structures in 3-D (e.g., directional couplers, splitters, lasers, etc.), there are also examples of longer pulse duration lasers (<1 microsecond) being successfully applied to 3-D fabrication of volume grating, for example (J. Zhang, P. R. Herman, C. Lauer, K. P. Chen, M. Wei, 157 nm laser-induced modification of fused-silica glasses, in Laser Appl. in Microelectronic and Optoelectronic Manuf. V, SPIE Proc. 4274, Photonics West, 20-26 Jan. 2001, pp. 125-132) or buried optical waveguide (for example, see M. Wei, K. P. Chen, D. Coric, P. R. Herman, J. Li, F2-laser microfabrication of buried structures in transparent glasses, Photon Processing in Microelectronics and Photonics, SPIE Proc. 4637, Photonics West, 20-25 Jan. 2002, p. 251-257) formation in bulk optical materials also without pre-existing waveguides.
The above methods demonstrate laser approaches for fabricating and integrating optical functions in 3-D that extend beyond optical fiber one-dimensional (“1-D”) and planar lightwave circuits (2-D). However, despite the work described above, there have only been limited attempts to inscribe grating structures into such laser-written waveguides.
In “Direct laser written waveguide-Bragg gratings in bulk fused silica,” Opt. Lett. 31, 2690-2691 (2006), Marshall et al. demonstrate a two-step laser method for writing second order grating structure in fused silica glass. A continuous waveguide is fabricated by scanning a focussed short pulse laser beam in bulk glass, then followed with the same laser in a different focusing condition to form Bragg gratings by point-by-point exposure that overlaps the newly-formed waveguide. Reflection spectra revealed a weak Bragg grating response while transmission spectra were not reported.
Yamaguchi described in Japanese Patent Application No. (2000)-144280 a method to generate an optical waveguide in doped glass with first-order Bragg gratings responses. The Bragg responses are induced during laser scanning by periodically changing the intensity of the laser light, the diameter of laser light at the focusing point or the relative moving speed. Smooth waveguides with periodic modification of refractive index are described. The method includes delivery of more than 100 laser pulses per waveguide segment (period typically of 0.5 μm) and weak period perturbations in the laser exposure conditions. The average intensity for forming 1300 nm wavelength Bragg resonances was varied from 90-100% during scanning using variable neutral density filters or shutters for attenuation.
In view of the foregoing, an improved means of forming waveguides simultaneously with Bragg gratings is desirable.